FIG. 1 illustrates schematically a capacitively coupled radio-frequency (RF) gas laser discharge. There is a significant electrical structure in the transverse (inter-electrode) direction, which is discussed in detail by D. R. Hall and H. J. Baker, "Slab Waveguide Carbon Dioxide Lasers", Japan Society of Laser Technology, 20(4), p31-52, October 1995 and Y. P. Raizer et al, "Radio-Frequency Capacitive Discharges", CRC Press, Fl, 1995, both of which are herein incorporated by reference.
In gas slab lasers, a high concentration exists of positively charged ions close to each electrode 2, 4. These are referred to as the ion-sheath regions 6, 8, and have a considerable influence on the discharge characteristics. Between these two sheaths is the plasma region 9, where the bulk of the laser excitation occurs. It has been known for many years that an alpha-type discharge characterized by a stable and positive impedance can provide efficient excitation of the upper laser levels in a gas discharge laser. The role of the sheath regions is believed to be critical in providing an effective stabilizing ballast for the plasma region. If for some reason conditions evolve where the sheath region is eroded in some way, then the stability of the alpha-type discharge is threatened and transition may occur to a gamma-type form of RF discharge. In extreme cases, and particularly with the application of high power RF, the onset of the gamma-type discharge can precede the collapse of the entire discharge to an arc. This usually results in catastrophic laser failure.
An important feature in the design of gas slab lasers is the formation of a rectangular discharge gain medium which is extremely uniform in lateral and longitudinal direction. The sheath voltage can be very large (e.g. 50-150 V.sub.rms), with a correspondingly low plasma reduced electric field. This field can be as low as approximately 0.4 V.sub.rms /cm. The discharge impedance can be very low, typically a few ohms. The high electric field in the ion sheath gives rise to energetic electrons (50-150 eV), which penetrate into the plasma region and generate an electron temperature suitable for the formation and population of the upper laser levels.
The ion sheath properties can enhance laser performance at lower RF frequencies such as 10-150 MHz. However, the freedom to select lower frequencies is typically limited by the need to ensure that the discharge is confined to the space between the electrodes over the full range of RF input powers, without escaping over longer paths to the surrounding grounded surfaces. This is easily achieved at frequencies above 150 MHz, but at lower frequencies this becomes progressively more difficult. These frequencies tend to be the optimum frequencies for maximum laser power extraction, however.
It is known to provide inductors between the electrodes at intervals along their length to alter the characteristic impedance of the RF stripline formed by the electrodes. The inductor values can be chosen to minimize the longitudinal voltage variations and hence non-uniformities in the discharge that arise from the RF standing waves.
FIG. 2 shows a typical circuit for an RF-excited gas discharge. Examples can be seen in Griffith (U.S. Pat. No. 4,352,188) and Chenausky (U.S. Pat. No. 4,363,126 and U.S. Pat. No. 4,443,877), each of which is herein incorporated by reference. An RF power source 10 is fed to the discharge electrodes 12 via a coaxial cable 14, an impedance matching network 16, and an RF vacuum feed-through connector 18 to the center of the electrodes 12.